The Los Angeles County Department of Public Health has identified the first cases of human West Nile virus infection in Los Angeles county for the 2025 season.

Four persons were hospitalized with West Nile virus illness beginning in late July through late August and are recovering. The identified cases reside in the Antelope Valley, San Fernando Valley and central Los Angeles areas.

West Nile virus spreads through the bite of an infected mosquito. Symptoms may include fever, headache, nausea, body aches and a mild skin rash. West Nile virus can affect the nervous system and result in meningitis, encephalitis, paralysis and even death.

While everyone is at risk for West Nile Virus, adults over the age of 50 years and people with chronic health problems are at higher risk of severe illness. Although not all mosquitoes carry this virus, the type of mosquito that spreads this virus is found throughout Los Angeles County.

“The first human cases of West Nile virus are an important reminder that we all need to take steps to prevent mosquito bites and mosquito breeding,” said Muntu Davis, MD, MPH, Los Angeles County Health Officer. “Mosquitos thrive in hot weather, increasing the risk of bites and mosquito-borne diseases. People should follow these simple steps to reduce their exposure to mosquitoes and risk for mosquito bites: 1) Use insect repellent on you and your family, 2) Remove items that hold standing water around your home where mosquitoes can lay eggs and breed and 3) Use, install, or repair window and door screens to prevent mosquitoes from entering your home.”

Public Health reports human cases of WNV every year in L.A. county, at an average of 56 cases per year during the last five years. However, the total number of people infected with WNV each year in L.A. county is much higher as most infected persons do not experience any illness or only mild illness. These cases are neither reported nor recognized as West Nile virus. About three-quarters of reported cases have had severe disease and approximately 10% of patients with severe WNV die from complications. This does not include cases in Long Beach and Pasadena as cases identified in those cities are reported by their local health departments.

Public Health recommends the following actions to reduce the risk of West Nile virus infection:

Avoid getting mosquito bites.

Avoid mosquito-infested areas at dawn and dusk as this is when mosquitoes that spread WNV are most active.

Use insect repellant. Use Environmental Protection Agency registered repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women. Find the right insect repellent for you by using EPA’s search tool.

Cover up. Consider wearing long-sleeved shirts and long pants when you are outdoors, particularly at the times and in areas where more mosquitoes are present.

Keep mosquitoes out of your home.

Use tight-fitting screens on windows and doors. Check for and repair holes in screens to keep mosquitoes outdoors.

Prevent mosquito breeding.

Eliminate standing water where mosquitoes can lay eggs. Once a week, empty and scrub, turn over, cover, or throw out items that hold water, such as tires, buckets, planters, toys, pet bowls, flowerpot saucers, rain barrels, or other containers. These are where mosquitoes lay eggs.

Empty and wash birdbaths and wading pools weekly.

Clean and chlorinate swimming pools; drain any water collecting on pool covers.

Stock garden ponds with mosquitofish, goldfish, Koi or other mosquito-eating fish. These feed on mosquito eggs and larvae.

Public Health collaborates with local vector control agencies to target areas for mosquito control activities and engage residents to provide information about how to protect themselves. There is no specific treatment for West Nile virus disease and no vaccine to prevent infection. Preventing mosquito breeding and mosquito bites are the best ways to prevent WNV infection. West Nile virus spreads during warm weather months when mosquitoes are most active. The virus survives in nature in several types of birds and is transmitted by the bites of mosquitoes that feed on infected birds. The emergence of WNV cases each year can be preceded by an increased number of dead birds.

“Detecting West Nile virus in our district is a reminder that this virus has been present in California, and right here in our community, for over 20 years. It’s part of our local environment and continues to reappear year after year. With the recent rains and more in the forecast, conditions are ideal for mosquitoes to breed,” said Brenna Bates-Grubb, Community Outreach Specialist, Antelope Valley Mosquito and Vector Control District. “We urge everyone to take simple steps to protect themselves and their families: use EPA-approved mosquito repellent, keep swimming pools clean and maintained, drain any standing water around your home and check for sources like buckets, plant saucers, or clogged gutters that can fill with rainwater. It only takes a small amount of water for mosquitoes to breed.”

For more information on West Nile virus, visit http://publichealth.lacounty.gov/acd/vectorwestnile.htm. To find a local vector control district, visit http://www.socalmosquito.org.

Neglected swimming pools or “green pools” that may contribute to mosquito breeding should be reported to the Public Health Environmental Health Division at (626) 430-5360 or online at http://www.publichealth.lacounty.gov/eh/safety/public-pools.htm, or to a local vector control district for pools at single family homes. Dead birds may be reported by calling (877) 968-2473 or online: https://westnile.ca.gov/report.php.